Austenitic chromium-nickel stainless steels resistant to stress-corrosion cracking



United States Patent 3,159,479 AUSTENETHC CHRQMEUM-NKCKEL STAINLES TEELS RESETANT T8 ETR'ESS-CQERRQSEQN CRACKENG Harry R. Copson, Crani'ord, and Frances S. Lang, White House Station, NJL, assignors to The international Nickel Company, inc, New York, NY, a corporation of Delaware No Drawing. Filed Nov. 28, N62, Ser. No. 240,736 4- Claims. (Cl. 75-428) The present invention relates to austenitic stainless steels and, more particularly, to overcoming the vexatious problem of stress-corrosion cracking in hot concentrated chloride solution to which the austenitic chromium-nickel stainless steels have been so susceptible heretofore.

The austenitic chromium-nickel stainless steels have been adopted for a vast number of different industrial and commercial applications, a result undoubtedly attributable, inter alia, to their relatively high strength properties at both room and elevated temperatures combined with their inherent ability to be fabricated with relative ease and their resistance to various.corrosive environrnents. With respect to corrosion resistance, there appears to be general agreement that the ability of such steels to resist corrosive media stems from their passive nature, i.e., films (generally chromium oxide films) form on the surface of the steels and these films tend to protect the surfaces from penetration by corrodents or to at least greatly inhibit corrosive attack. Unfortunately, these passive films are under certain environments too easily ruptured or removed.

Corrosive activity of austenitic nickel-chromium stainless steels has been classified intofour major categories: (1) general corrosion (usually a complete breakdown of the passive or protective film), (2) pitting corrosion (generally the occurrence of localized attack coupled with depth of penetration), (3) stress-corrosion cracking and (4) intergranular corrosion. The present invention is particularly concerned with the prevention of stress-corrosion cracking, a condition which generally arises Without ally significant occurrence of general corrosion.

As is indicated in the authoritative treatise, Metals Handbook, SthEd, (1961), p. 566, and by practically all other authorities who have grappled with the problem, all commercially available austenitic stainless steels are subject to stress-corrosion cracking. This problem has existed for at least twenty-five years and has been thought to have existed for a longer period except that it went unrecognized as such by earlier investigators. Notwithstanding the numerous articles or treatises (there are at least 250 references to the problem) which have been written concerning the subject, there is no single generally accepted theory which explains the Why of this phenomenon. To be sure, the theories advanced have been numerous and the following are illustrative of but a few of'them: (1) the quasirnartensite theory, i.e plastic deformation results in the formation of a quasi-martensite which is preferentially attacked so as to result in cracking, (2) the plastic deformation theory which involves the. formation of crack-sensitive paths wherein precipitates (carbides'and nitrides) are formed at stressed areas which are thought to act as cathodes and the contiguous area of the steel asanodes of electrochemical cells entering into progressive corrosion and. crack propagation, (3) the restricted slip theory whichessent-ially is a theory of predieting susceptibility tostress-corrosion cracking, i.e., alv loys which manifest cross slipping rather easily are considered to possess immunityto cracking, whereas alloys which inherently exhibit a high degree of resistance to slip are considered to be susceptible to cracking. The

last might be consideredsomewhat as a special case of 3,l59,l79 l aten'teel Dec. l, 1%54 ice one of the two most prominent broad theoretical explanations presently in vogue, to wit, the periodic electrochemical-mechanical mechanism. There are other explanations of the problem such as those involving surface films or involving dislocations, but no one mechanism has achieved general acceptance.

While there may be no single theory which conclusively explains the fundamental nature of stress-corrosion cracking, it can be stated with assurance that if an austenitic chromium-nickel stainlesssteel is under stress, whether the stress be externally applied and/ or residually induced by some processing operation such as quenching or cold-working, and the steel is exposed to certain corrosive media, the conditions are ready-made for failure by stresscorrosion cracking. Chloride solutions, as indicated in the Metals Handbook, are the worst offenders and these include the chlorides and other halides of magnesium, calcium, lithium, sodium, etc. Some other solutions have also been known to promote stress-corrosion cracking.

As there have been a number of theories to explain I nection, the use oi ferritic stainless steels has been given consideration since the ferritic types are ostensibly rather immune to stress-corrosion cracking. To do so, however, would be to lose all the advantages of the austenitic stainless steels. It has also been suggested to avoid contact between austenitic stainless steels and chlorides. This is hardly a panacea, although it might circumvent the problem. The use of inhibitors, e.g., chromates, phosphates, and oxygen scavengers, has been urged, but this approach has not proven successful and brings into play other undesirable problems. Changes in design, processing conditions and/or fabrication have been proposed. For example, the use of high temperature stress-relieving treatments has been advanced in an effort to minimize the effect of residual stresses, it being considered that residual stresses are more causative of stress-corrosion cracking than stresses externally applied. This approach has not proven to be the answer, partly because itis difficult to reduce and 'to maintain the stresses at a low level. Another proposal is the use of the wellknown cathodic protection principle. This is based upon the consideration that stress-corrosion cracking is of an electro chemical nature and the use of protective currents should assist in preventing cracking. The difficulties arising from this approach including the impracticability of carrying it out on a commercial scale, are well known and will not be dwelt upon herein. In addition, stress-corrosion cracking still occurs. Other proposals include change of alloy composition, e.g., maintaining the carbon and nitrogen contents at very low levels in 18-8 stainless steel and cold working. Actually, this results in a ferritic stainless steel or one very high in ferrite.

Of all proposals, considerable interest has been shown with regard to purity of composition with emphasis on keeping the nitrogen (av known subversivepe'rformer) crease the cost throughthe use of high purity materialsv and expensive vacuum treatments would be quite disadvantageous. In addition, there is considerable conflict a? as to whether purity is the answer. In any event, a more practical solution is required and it is to this aspect that the instant invention is directed.

Although many attempts were made to overcome the problem of stress-corrosion cracking in austenitic chromium-nickel stainless steels, none, as far as we are aware, was entirely successful when carried into practice commercially on an industrial scale. This includes those proposals involving the beneficial effects of nickel, carbon or silicon and low amounts of nitrogen.

It has now been discovered that the problem of stresscorrosion cracking of austenitic stainless steels can be overcome or greatly minimized while utilizing standard air-melting techniques (as opposed to expensive vacuum techniques) and commercially pure grades of materials (as opposed to high purity materials) provided the steels are of specially controlled composition as set forth hereinafter. It has been further found that certain elements, notably phosphorus, promote the susceptibility of austenitic stainless steels to stress-corrosion cracking. However, in accordance with the invention, the presence of controlled amounts of phosphorus is not harmful and can be readily tolerated, thus avoiding questionable recourse to the use of pure materials or to undesirable processing techniques to rid the steels of phosphorus. In addition, because of the specially balanced composition of the steels within the invention, larger amounts of nitrogen can be present than otherwise might be the case.

It is an object of the invention to provide austenitic chromium-nickel stainless steels which greatly resist stresscorrosion cracking notwithstanding that the steels are highly stressed and in contact with corrosive solutions, including chloride solutions, which are known to be causative of stress-corrosion cracking.

Generally speaking and in accordance with the present invention, it has been found that stress-corrosion cracking of austenitic chromium-nickel stainless steels can be eliminated or greatly minimized with steels of the following most advantageous composition: about to about nickel, about 16% to about 25% chromium, at least 0.07% and up to 0.09% carbon, up to about 1.5% silicon, phosphorus in an amount up to not more than 0.015%, nitrogen in an amount up to 0.035%, the sum of the phosphorus plus nitrogen being not more than 0.045%, and the balance essentially iron. Phosphorus, among other elements, has been found to exert a pronounced adverse influence in the resistance of austenitic chromium-nickel stainless steels to stress-corrosion cracking. However, provided that the phosphorus content is controlled in accordance with the invention and provided the composition of the steels is otherwise critically balanced, the detrimental effect of phosphorus is overcome. This avoids use of processing techniques to eliminate phosphorus and is thus beneficial in maintaining the cost of producing the steels at an economic level. In addition, it has been found that what might have heretofore been considered as a relatively high and detrimental amount of nitrogen, e.g., 0.035%, can be present in the austenitic steels of the instant invention provided that the composition is critically balanced. A particular feature and advantage of the invention is that air-melting practices and commercial grades of materials can be employed. These factors greatly contribute to maintaining the economic competitiveness of the steels.

Satisfactory results can also be achieved with austenitic stainless steels of the following composition: at least 19% and up to about nickel, about 15% to about 30% chromium, at least 0.07% and up to not more than 0.1% carbon, up to about 1.6% silicon, phosphorus in an amount up to not more than 0.018%, nitrogen in an amount up to 0.045% but preferably not greater than 0.04%, the sum of the phosphorus plus nitrogen being controlled such that it does not exceed 0.055%, and the balance essentially iron.

In carrying the invention into practice, it is most irnportant that special attention be given to controlling the nickel, carbon and phosphorus contents. As will be demonstrated hereinafter, should the nickel content fall below 19%, e.g., 18%, stress-corrosion cracking can occur. Amounts of nickel higher than 35% are unnecessary and only increase cost. The minimum carbon content is exceedingly important. As will become clear from data presented herein, carbon contents of 0.06% and lower are unsatisfactory and result in stress-corrosion cracking in air melted austenitic chromium-nickel stainless steels made from materials of commercial purity. Carbon above 0.1% intensifies susceptibility to intergranular corrosion. Above 0.1% carbon, too great an amount of chromium is taken up in the form of chromium carbides. This depletes the grain boundaries of the austenitic steels of chromium and promotes greater susceptibility to intergranular corrosion. It might be considered that carbide stabilizers such as columbium and titanium might be employed to counteract this effect. The ditfculty here is that the columbium and/ or titanium would tie up the carbon (columbium and/or titanium carbides would preferentially form as opposed to chromium carbides) to the extent that while the steels might be immune to intergranular corrosion, they would be susceptible to stress-corrosion cracking. The effort would be self-defeating. Phosphorus, a subversive promoter, should not exceed 0.018%. It has been found, for example, that even in high-purity vacuum-treated austenitic stainless steels, 0.023% phosphorus is most harmful since it actively promotes stress-corrosion cracking. As those skilled in the art will appreciate, the A131 commercial grades of austenitic stainless steels contain up to 0.2% phosphorus. Phosphorus contents of the order of 0.2% and higher have been known to impart beneficial effects to austenitic stainless steels. For example, it has been indicated that even 0.12% phosphorus improves stress-rupture properties at high temperatures. However, in accordance with the invention, such amounts of phosphorus are detrimental. ,Nitrogen is present in austenitic chromium-nickel stainless steels and is known to act subversively. There are prior references indicating that nitrogen should be maintained at a very low level; however, it has now been found that in the steels provided in accordance with this invention, comparatively high amounts of nitrogen, i.e., up to 0.045 can be present. However, the total phosphorus and nitrogen contents should not exceed 0.055% and it is most advantageous that the sum not exceed 0.045 Additionally, if the phosphorus content is on the high side, it is preferable to maintain the nitrogen content on the low side and vice versa.

For the purpose of giving those skilled in the art a better understanding and/or appreciation of the invention, there is given hereinafter data which are illustrative of the advantages embodied by the stainless steels having compositions within the invention. However, it should be mentioned that the test employed in connection with the data, to wit, immersion of stressed U-bend specimens in boiling concentrated magnesium chloride, has been criticized as being too severe on the grounds that such severe conditions would not be encountered in usual commercial applications. In addition, the tests conducted in accordance with the invention have been continued for longer time periods than are usually considered necessary. If the stainless steels will withstand what is considered to be a severe test, then it would seem that the results thereby obtained would be a more critical indicator as to reliability, particularly from the commercial viewpoint.

In addition to using air melting practice in combination with commercial grades of materials, e.g., Armco iron and ferrochromium, specimens for test were also repared using materials of high purity (electrolytic iron electrolytic chromium) and vacuum melting technique. Because of availability, carbonyl or electro-nickel was used in the tests. Initially, ingots were prepared from compositions as set forth in the appropriate Tables hereinafter. The ingots were machined and ground to remove all surface defects and contamination, were heated in air to a temperature of about 2150 F. and were then forged in air to inch slabs. All surfaces were again machined to a depth of /s inch to insure removal of any contaminating effect that might have been introduced from the forging operations.

A number of test specimens were machined from the forgings and subjected to the U-bend, magnesium chloride test. Other specimens of the forged material were cut into inch x /2 inch slices and cold-rolled about 40% and subjected to test in the cold-rolled condition. In addition, cold-rolled specimens were also annealed in an argon atmosphere for one hour at 195 0 F. and water quenched. These too were subjected to the U-bend, magnesium chloride test. In some cases the forgings were hot-rolled prior to cold rolling. All specimens were finish ground to about 40 microinches or finer to avoid possible surface contamination. Still other specimens were sensitized, i.e., subjected to an intermediate temperature treatment, e.g., about 1250 F., and then subjected to test in the sensitized condition. This was conducted with the view that austenitic stainlesssteels, as

a commercial proposition, are often subjected to the sotermed sensitization treatment (as by an intermediate heat treatment or welding operation) and this could affect susceptibility to stress-corrosion cracking. Thus, specimens were tested in at least one or more of four different conditions so as to intensify and diversify test conditions, thereby avoiding reliance on the possibility that stresscorrosion cracking might not occur in one condition with out knowing whether it would occur in one of the others.

The test specimens were formed in the configuration of a U-bend by taking specimen samples of about 6 inches long, /2 inch wide and about A; inch thick, and bending them over a mandrel using a tensile test machine. This induced considerable stress in the specimens and the specimens were then placed in a vise and the legs of the U-bends were drawn about parallel to. each other by tightening the vise. Bolts were inserted through solutions were used to replace the old solutions. Generally, if no stress-corrosion cracking was evident after a period of thirty days (720 hours), testing was suspended although in some instances it was continued for a longer period of time. After removal from the test solution, micro-examination Was made or" all specimens and the apices of the bends were examined in longitudinal cross-section near the surface and along the center line to confirm the presence or absence of the cracks.

As mentioned hereinbefore, it has been found that phosphorus is a potent promoter of the susceptibility of austenitic chromium-nickelstainless steels. This is true notwithstanding that highly pure materials and vacuum techniques are employed. Inthis connection, vacuum melting, electrolytic iron, electrolytic chromium and carbonyl nickel were used in the preparation of the austenitic stainless steels having compositions shown in Table I. i

TABLE I Chemical Composition Per- Per Per- Per- Per- Per- Alloy N 0. cent cent cent cent cent cent Ni Or 0 P 20. 6 16. 3 0. 001 0. 01 0. 003 0. 0001 20. 3 17. 1 0. 004 0. 04 0. 002 0.0001 20. 8 l8. 1 0. 003 0. 06 0. 003 0. 0032 20. l 17. 9 0. 002 0. 01 0. 023 0. 0048 20. 3 l8. 1 0. 003 0. 01 0. 065 0. 0004 21. 0 18. 8 0. 006 0. 02 0. 35 0. 0007 46. 8 17. 9 0. 003 0. 18 0. l3 0. 0537 The results of the test are given in Table I-A. The period to crack is given as of the day cracking was observed, but since inspection was usually made every second or [third day, cracking could have taken place before the. day indicated. For example, if cracking time is given as 15 days, cracking could have occurred between the 13th and 15th day. In addition, failure of any specimen in this multiple test (including other results given herein) is taken as showing that the stainless steel involved is unsatisfactory and is not in accordancewith the invention.

TABLE I-A Cracking Time A e Alloy Forged ColdRolled Cold Rolled sensitized Percent ni i No. and Annealed Cracked Crack in Days 30 OK OK'OK 15 15 OK OK 3 41130---- as 215 11 OK OK 21 21 21 2s 10 20 30 30 s3 20 21 0K 01; OK 24 on OK OK 21 21 2s 30 25.4

122336OK6666- 92 6 11111111111-- 100 1 1111,11-11111 100 1 33seaee3a3s 100 4 holes near the ends of the legs of the specimens. Bcil- The data in Tables I and I-A illustrate the detrimental ing concentrated magnesium chloride was used as the effect of phosphorus under what might be considered as corrosive agent because of its known severity in promotthe most favorable conditions, to wit, vacuum melting ing stress-corrosion cracking. The solutions were of d the use Of eXifelIlelY P materialswhen the about 42% concentration and the boiling point was ad- Phosphorus 001K611? was rather low as n Alloys 2 an justed to 154 0. Five liter flasks equipped with reflux 3, The average hfeetmrlack Was comparatlvely s condensers held the magnesium chloride. Four test speci- However, as the phosphorus content was mens of at least one of the groups (forged, cold-rolled, I, Increased cracks became 3?? almost lmmedlatiily' annealed or sensitized): under test were suspended in For fi gm f 8 g F each flask with the apices of the bends and about half $32 g 1 YthNQ. gontalnlgjg of the legs immersed in the solution. The time required 0 p p Orus crac i 658 an for crackin was the criterion em 10 ed in determinin gardmg Alloys 1 and It can be Stated'that these fbiug a t g au'stenitic steels were of high purity; neyerthelessythey suscep 1 y or M18 ance 0 s less-corrosion :11gcracked and this is indicative that the useof high purity The were u removed from evgry materials alone in producing austentic chromium-nickel Second third y m e under a Infiignlfifir for steels will not provide resistance to stress-corrosion crackcracks. When cracking was evident, the speclmens were i as mntemplated by h Present invention I removed from test. When no cracking occurred m a Not only must the phosphorus content be controlled, period of about two weeks, fresh magnesium chloride but, as mentioned above herein, the alloy composition similarly, the steels oys Nos. 21 and 22) Increasing the in steels containing to be noted that /6 It is much 8 Table H-A reflects that all the austenitic chromiumickel stainless steels containing less than 0.07% carbon (Alloys Nos. 8 through 17, and 22 through 25 and 7) cracked regardl ll egardless of carbon content.

containing less than 19% nickel (A- cracke nickel content from 21.71% to 46% less than 0.07% carbon (Alloys Nos. 23 through 25 and Alloy N0. 7) did not prevent the occurrence of cracking within the 30-day period. It is also or the specimens of Alloy N0. 28 cracked and this alloy contained a total of 0.063% of phosphorus plus nitrogen. No cracking occurred in Alloy No. 29 which was otherpreferred that the total content of phosphorus and nitrogen be controlled such that it does not exceed 0.055% and prefera in Table II were subgiven TABLE II Chemical Composition 1 as a whole must be critically balanced. This is particularly apropos regarding the nickel, carbon, nitrogen or nitrogen plus phosphorus contents. As will be seen from data presented hereinbelow, small amounts of such elements can greatly affect the results obtained. In this 5 connection, several austenitic chromium-nickel stainless steels having compositions jected to test and the results are given in Table II-A.

It might be pointed out that in the main Alloys Nos. 8 through 20 reflect the importance of a minimum carbon 10 content of 0.07%, Alloys Nos. 21 and 22 illustrate the efiect of nickel contents below 19%., Alloys Nos. 23 through 26 and Alloy No. 27 show the effect of high nickel contents in austenitic alloy steels otherwise outfurther indication of the necessity of maintaining a proper correlation in the amounts of nickel, carbon, phosphorus hC eOV S .rt L m mm n w mnmmw .mwmmwmm a t U. f .S mmfn flntmom wmmm w ww W m lwi me e 0 rs .n 0 d u e t O n a n O r y W m mwmw me maem a m n u e .mllptkeo mt osha n f Om m mn M. .f7 m.1 fi nment m v .fn n% wM a L .meo mw en w nn c c n l. a 0 a m m mfimwnommwnznooonv 729580 .0. n a n n an m s m s v s n s u t k r u 0 a an nwnnnnmnn n mnnnn mnmm n a wnmd m k rt wn mm0 mn mn m e ALo re..m mmmzm wm nmmmamm i 5 6 Yn u .10 O 5.1 3 b n m to e n t a n m :7 an n m m a m 0 e a n mmmmwm mw wnmmmw m t wmo mn m fioww uMmu W Ma 111111 11111 r M HdM w w oe mmm nb omw P wt mm mmm wm m m n n 0 m m w a a advfmme KK K KK U KKKKK stun. e. n d dfi m e 00000 00000 m anflmrfiscma w w aiummm 6 f. i 0: .1 O n n 1 .lfs a en n, C TS d 0 1m dem.e mc n mm%ns m E f nxiixnn lnnxan m m wwmw/ m m l n 00000 00000 I n S Q mk wd RU M O n 255 2KKKKK11447 KK n ne a o onv n m el s o "N 11 Mt.rmwmamour/tmommasmmnmvnimooae 00000 00000 T s a a I mm mm mm mm. sagneme E Enniinnn lxxnnx l iT k%h nn i m m u mw l l m omu omcm 000 00000 t 1 6 b 6 1. O .aa 2. F mnmm m m M a a aaea a n a i r n d 00 ctciblaaPcOnooO aOcAQin n a d a a a .0. A aaae ma e 6 M t 7 tn St H 2 18 m wmmmwmem mmmmmmmwmmmmmma m n m ml n anKn iw nKnnK w onnuooonnnnonooooononooo um mm E T m 00 00 0 00000 c C y .m 00 126677 99 O45573C m mnmwwmwwmowmwmwmmmnmmomm mm mm k 00 0 0 00000 I C PW Q0000.0.0.000.0.0.0.0.0.0.00.0.0.0.00.0. mm 0 0m T m 1ZQEW7KKKKKKK$K5HW6KKKKK t w mm C 0000000 0 00000 i Lm .wnnnnmnnmmnnnwmnnnmnnmnm mm m izvsl r a szoscee W ooduuodooddooarodoaraood mm M 12 m 13 MMMWW t 0 t 8 .1 d n 5333 0 1 1 KK KKKK% KKKKK m mmmwnnnnwmmwnnwmmmmnnnn mm m 11 00m0 o0 3 13 00000 a aoaaooaooaoaaoanaanaodao .nm Sn 0. mm o KKKKKKKNH NW KKKKK m e N 0000000 00000 e 88515380255076984907771 rw t m 7 7 7 &&ZZ&&7 Z7 Q7 ZZ7 7 7 &ZZZ& A .mD m: -K any. .111111111111111111111111 m 2 .mw x t I S 10 m nan fiiee wnnfiemnnsneenb wmmmw m n E K :1 I o mmmmmm mmimm m 0 D. m mm 0 mw mm F m m-K h a O anm m o m mmn m m m. K N u o Wmm O w .ammipm 0 u H .Namw y A MSRH .n 0 n l owi UN mmmmnm A m m 1 8 AB GM 8 side the invention and Alloys Nos. 27 through 31 are a wise and nitrogen.

h TABLE III Chemical Composition Alloy Percent Percent Percent Percent Percent Percent Percent N o. N Si P N A 8: 8? 8: 8: gi nominal com-positions, but, as referred to before herem, 18,3 0,05 1 0,0005 1nd1cates that they suffer from stress-corrosion crack- 16.9 0. 04 0. 42 0.012 0. 033 0. 05 m 16.4 0. 00 0.03 0.013 0.037 0. 04 17.7 0.03 0. 04 0. 012 0.043 0.1 Although the present invention has been described in conjunction with preferred embodiments, it is to be TABLE III-A Cracking Time Alloy Cold Rolled and Percent Average No. Forged Cold Rolled Annealed sensitized Cracked Life-to- Crack in Days K OK OK OK 9 9' 30 30 13 14 30 OK 53 23.3 2222446OK222OK--'- 33 7.3 -14 14 14 30 9 920 220K0K0K0K, 07 21 14.21212113131313 3 6 7 7 100.120 OK OK OK OK 15 29 OK OK OK OK 33 27.3 -OKOKOKOK OKOKOKOK OKOK OKOK 0 30- Alloy No. 33, which was prepared with high purity understood that modifications and variations may be rematerials and vacuum treated (as was Alloy No. 32), sorted to Without departing from the spirit and scope showed a considerable decrease in resistance to stressof the invention, as those skilled in the art will readily corrosion cracking with an aluminum content of 0.04% 30 understand. Such modifications and variations are conas again-st Alloy No. 32 which contained 0.01% alumisidered to be within the purview and scope of the invennurn. A similar pattern of behavior is shown by Alloys tion and appended claims.

Nos. 34 through 36 and 29. It is to be particularly noted We claim:

that Alloy No. 36 which would b oth r i within th 1. An austenitic chromium-nickel stainless steel highly invention except that it contained 0.04% aluminum failed 35 resistant to stress-corrosion cracking when subjected to whereas Alloy No. 29 which is within the invention and Stress and ill Contact With halide Solutions, Said Steel quite similar in composition to Alloy No. 36 except that consisting essentially 0f about 20% i0 30% nickel, about it contained 0.1% aluminum completely resisted cracking 15% about 25% chromium, at least (107% and P to during the test. 0.09% carbon, up to 1.5% silicon, phosphorus in an With regard to other elements that can be present an amount p not mfife than nitrogen in all in the stainless steels contemplated in accordance with mount p $0110t more than (1035 e Sum of t e the invention, it is to be pointed out that manganese Phosphorus P nitrogen not excfieding 0-045%, p should not be present in an amount greater than 0.7% 07% manganese 911d the balance essentially ironand advantageously should not ex e d 02% if t a 2. The austenitic stainless steel set forth in claim 1 corrosion cracking is to be avoided. In addition, it is wherein the manganese content is not gr than preferred that copper should not be present in an amount An austenitic chromium-nickel Stainless Steel highly greater th 02% d i i more f rr d to maintain resistant to stress-corrosion cracking when subjected to copper t l th (11%, stress and in contact with halide solutions, said steel 1.; i m b b d h the pmsent invention consisting essentially of from 19% to about 35% nickel, vides austenitic chromium-nickel stainless steels highly .50 about 15 to about 30% chromium, at least (107% and resistant to stress-corrosion cracking when subjected to P to 01% carbon, 1 i0 15% Silicon, Phosphorus in stress and in contact with halide solutions. Thus, the an mount up to not more {Khan (101893, nitrogen in a steels, in addition to being highly useful for high temamount P to not more film the 811m of the perature applications, are especially suitable in the chemi- Phosphoms P nitrogen not exceeding 095% up 10 cal processing industry where stress-corrosion cracking 0 manganese, d he balance essentially iron. has heretofore been so prominent. It is to be fur- T austenific Stainless Steel set forth in claim 3 they pgintgd out h no impairment in mechanical whereln the manganese content is not greater than 0.2%. erties of the austenitic steels within the invention is I encountered. Actually, the mechanical properties show Referemes Quad m file of thls Patent an improvement over the properties of the typical A181 UNITED STATES PATENTS grades of austenitic chromium-nickel stainless steels.

3,118,761 Hull in. 24, 1964 UNITED STATESUPATENT CERTIFICATE OF CORRECTION Patent No. 3 ,l59,479 December 1 1964 Harry R. Copson et a1.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below Column 2, line 52, after "approach" insert a comma; column 7 line 13, for "Alloy No 27" read Alloy No. 7 same column 7, TABLE II, footnote 1, line 1 thereof, for "Allo" read Alloy column 10, line 7, for "compositions" read composition line 41, for "mount" read amount Signed and sealed this 3rd day of August 1965.

(SEAL) Attest:

ERNEST w. SWIDER i EDWARD J. BRENNER Aitesting Officer Commissioner of Patents 

1. AN AUSTENITIC CHROMIUM-NICKEL STAINLESS STEEL HIGHLY RESISTANT TO STRESS-CORROSION CRACKING WHEN SUBJECTED TO STRESS AND IN CONTACT WITH HALIDE SOLUTIONS, SAID STEEL CONSISTING ESSENTIALLY OF ABOUT 20% TO 30% NICKEL, ABOUT 16% TO ABOUT 25% CHROMIUM, AT LEAST 0.07% AND UP TO 0.09% CARBON, UP TO 1.5% SILICON, PHOSPHORUS IN AN AMOUNT UP TO NOT MORE THAN 0.015%, NITROGEN IN AN MOUNT UP TO NOT MORE THAN 0.035%, THE SUM OF THE PHOSPHORUS PLUS NITROGEN NOT EXCEEDING 0.045%, UP TO 0.7% MANGANESE AND THE BALANCE ESSENTIALLY IRON. 